1. Field of the Invention:
This invention relates to a photosensitive thermosetting resin composition and a method of forming a solder resist pattern by the use thereof, and more particularly relates to a novel photosensitive thermosetting resin composition useful as metarials for the production of printed circuit boards, the precision fabrication of metallic articles, the etching of glass and stone articles, the relief of plastic articles, and the preparation of printing plates and particularly useful as a solder resist for printed circuit boards and to a method of forming a solder resist pattern by the steps of exposing a layer of the resin composition selectively to an actinic ray through a photomask having a pattern and developing the unexposed part of the layer.
2. Description of the Prior Art:
The solder resist is a substance which is used during the soldering of a given part to a printed circuit board for the purpose of preventing molten solder from adhering to irrelevant portions and protecting circuits. It is, therefore, required to possess such properties as high adhesion, insulation resistance, resistance to soldering temperature, resistance to solvents, resistance to alkalis, resistance to acids, and resistance to plating.
The solder resist in the early stage of popular use was preponderantly of the epoxy melamine-based thermosetting type. The epoxy melamine solder resist betrayed deficiency in resistance to soldering temperature, resistance to chemicals, and resistance to plating. For use in the production of industrial-use printed circuit boards, epoxy-based thermosetting type solder resists have been disclosed in the specification of Japanese Patent Publication No. SHO 51(1976)-14,044, for example, as improved versions of the early solder resist mentioned above. At present, they are prevailing over those of other types. For use in the production of consumer-use printed circuit boards, since productivity dictates the dominant consideration, such rapid-setting ultraviolet setting type solder resists as disclosed in the specification of Japanese Patent Publication No. SHO 61(1986)-48,800 are prevailing now. The ultraviolet setting type solder resists, however, cannot be used in the production of industrial-use printed circuit boards because they pose a problem as to the setting property in the bottom of a thick film and betray deficiency in resistance to heat. These solder resists rely on the screen printing method for the formation of a solder resist pattern. In the formation of solder resist pattern which are expected to follow the recent trend of electronic equipments and devices toward reduction in weight and volume and the consequent trend of printed circuit boards toward increased density of integration and the trend of the industry toward adoption of the practice of mounting component parts on the surface of a printed circuit board, the ultraviolet curing type solder resists are disadvantageous in staining a pattern by bleeding and skipping between adjacent conductors and, therefore, are no longer capable of fulfiling the function expected of a solder resist film.
For the solution of these problems, dry film-type photo-solder resists and liquid photo-solder resists have been developed. As a dry film-type photo-solder resist, a dry film-producing photosensitive resin composition comprising a urethane-di(meth)acrylate, a linear polymer possessing a specific glass transition point, and a sensitizer is disclosed in the specification of Japanese Patent Application laid open to public inspection, KOKAI (Early Publication) No. SHO 57(1982)-55,914. When such dry film-type photo-solder resists are used in high-density printed circuit boards, however, they are deficient in resistance to soldering temperature and in adhesion.
As aliquid photo-solder resist, a photopolymerizing coating composition comprising a solid or semisolid reaction product of polyepoxide and an ethylenically unsaturated carboxylic acid, an inert inorganic filler, a photopolymerization initiator, and a volatile organic solvent is disclosed in British Patent Application Early Publication No. GB-2,032,939A. Since this composition uses only an ultraviolet curing component and has no use for a thermosetting component, it is deficient in adhesion to a printed circuit board, resistance to soldering temperature, and insulation resistance. As a version which pays due consideration to the thermosetting property in question, a solder resist ink-quality resin composition comprising the reaction product of a phenol-novolak type epoxy resin with an unsaturated monobasic acid, the partial reaction product of a cresol-novolak type epoxy resin with an unsaturated monobasic acid, an organic solvent, a photopolymerization initiator, and an amine type curing agent is disclosed in the specification of Japanese Patent Application KOKAI No. SHO 60(1985)-208,377. This composition is intended to utilize additionally the phenomenon of thermal setting by allowing an epoxy group to remain in the molecular unit. Since this retention of the epoxy group results in a partial decrease of the photosensitive group, the composition suffers from a decline in the ability to cure itself upon exposure to the ultraviolet light. Since the composition does not permit ample retention of the epoxy group, it is incapable of exhibiting fully satisfactory properties expected of a solder resist.
As versions which involve additional use of an epoxy resin, a photosensitive composition comprising an unsaturated compound containing at least two terminal ethylene groups, a polymerization initiator, a compound containing at least two epoxy groups, and a compound containing at least two carboxyl groups is disclosed in the specification of Japanese Patent Application KOKAI No. SHO 49(1974)-107,333 and an ink composition comprising a photosensitive prepolymer obtained by causing the reaction product of a novolak type epoxy compound and an unsaturated monocarboxylic acid to react with the reaction product of a diisocyanate and a polyfunctional (meth) acrylate containing one hydroxyl group in the molecular unit thereof, a photoinitiator, and an organic solvent in addition to an epoxy resin is disclosed in the specification of Japanese Patent Application KOKAI No. SHO 61(1986)-272. While the latter composition exhibits to a certain extent properties expected of a solder resist, the former composition is inferior in resistance to soldering temperature and resistance to solvents because it is based on a (meth)-acryl group-containing acryl type linear polymer. These compositions are both of such quality that when their epoxy resin contents are increased, their photosetting property or so-called sensitivity is degraded and their resistance in the exposed portion thereof to the action of a developing solution tends to decline even to a point where they no longer endure protracted development and tend to induce incomplete development of an unexposed part. A resist ink composition comprising a photosetting resin obtained by the reaction of a saturated or unsaturated polybasic acid anhydride with the reaction product of a novolak type epoxy compound and an unsaturated monocarboxylic acid, a photoinitiator, and a diluent in addition to an epoxy resin is disclosed in the specification of Japanese Patent Application KOKAI No. SHO 61(1986)-243,869. This composition requires use of an aqueous alkali solution as its developing solution. Therefore, if the content of epoxy resin possessing no solubility in the aqueous alkali solution is increased, the composition similarly suffers from degradation of its sensitivity and decline of its solubility of the unexposed portion in the developing solution to a point where the unexposed portion remains undeveloped and the development is required to be performed for an unduly long time and the exposed portion is corroded by the developing solution.